Method of producing heat exchange elements



W. P. HILL 7 METHOD OF PRODUCING HEAT EXCHANGE ELEMENTS Feb. 19, 1952 2SHEETSSHEET 1 Filed June 22, 1951 FIG.2.

INVENTOR.

WALTER P. HILL Feb. 19, 1952 w. P. HILL METHOD OF PRODUCING HEATEXCHANGE ELEMENTS Filed June 22, 19511 2 SHEETS-SHEET 2 FIG.7.

INVENTOR.

WALT ER P. HILL wyw ATTORNEYS Patented Feb. 19, 1952 METHOD PRODUCINGHEAT EXCHANGE ELEMENTS Walter P. Hill, Pontiac, Mich., assignor toCalumet and Hecla Consolidated Copper Company, Calumet, Mich., acorporation of Michigan Application June 22, 1951, serial No. 238,043

2 Claims. 1

This invention relates to an improved method of producing a heatexchange element and is a. continuation-in-part of my co-pendingapplication, Serial No. 42,048 on Heat Exchange Device and Method ofForming the Same."

One of the objects of this invention is to provide a heat exchange unitcomprising an outer finned tube and an inner tube secured in intimatecontacting relation with the outer tube to form a liner for the latter.

Another object of this invention is to provide a heat exchange unitwherein a helical fin is integrally formed on the outer surface of theouter tube and wherein the inner surface of the outer tube is pressedinto intimate engagement with the outer surface of the inner tube by thefin forming operation.

Still another object of this invention is to mechanically interlock thetwo tubes together by the fin forming operation in a manner such thatany heat loss resulting from the joint between the tubes is negligibleeven where extremely high temperature differentials are encountered.

A further object of this invention is to provide a composite tubularstructure of the type noted above wherein the outer tube may be formedof one metal and the inner tube may be formed of a different metal. Forexample, the outer tube may be formed of a white metal such as aluminumand the inner tube may be formed of copper or steel. Thus, the part ofthe composite tubing from which the fins are extruded may be formed ofan inexpensive, ductile material capable of being easily worked duringthe fin forming operation which, of course, is desirabl in that itreduces tool breakage as well as cost to a minimum. At the same time theabove arrangement enables employing a copper or copper alloy linercharacterized in that it resists corrosion by fluids ordinarily used,for example, in refrigeration and renders it possible to obtain highlyeffective brazed joints with such materials as silver solder forexample. Moreover, since the two tubes are pressed into intimateinterlocking relationship during the fin forming operation, the aboveadvantages are obtained without an appreciaable loss in heat transferefliciency. In actual use of such a composite finned tube, the loss maybe five per cent or less which, in most installations, is insignificantespecially when the saving in cost is considered.

The foregoing as well as other objects will be made more apparent asthis description proceeds, especially when considered in connection withthe accompanying drawings, wherein:

Figure 1 is a longitudinal sectional view through a. composite finnedtube or heat ex= change device enlarged approximately three times itsactual size;

Figure 2 is a. cross sectional view taken substantially on the planeindicated by the line 2-2 of Figure 1;

Figure 3 is a semi-diagrammatic cross sectional view through one type ofapparatus that may be employed for forming an integral helical fin onthe outer tube of the heat exchange device;

Figure 4 is a sectional view taken substantially on the plane indicatedby the line 4-4 of Figure Figure 5 is a greatly enlarged fragmentarycross sectional view showing one of the roll sections in fin formingrelationship with the outer tube;

Figure 6 is a sectional view taken substantially on the plane indicatedby the line 6-6 of Figure Figure 7 is an enlarged fragmentary sectionalview illustrating a part of one of the rolls in operative relation tothe outer tube; and

Figure 8 is a fragmentary sectional view of the plain tubes prior tobeing subjected to the fin forming operation.

The reference numeral 10 in Figure 1 of the drawings designatesgenerally a heat exchange device which may take any shape required forthe particular use to which the device is to be put. For example, in theevent the device is to be used as either a condenser or an evaporator ina refrigerating system, the device may be bent into a flat or helicalcoil or may be fashioned to a serpentine contour.

In any case, the device comprises an outer tube H and a liner or innertube If. The outer tube has a helically extending fin l3 integral withthe outer surface thereof and is also provided with a helicallyextending groove H in the inner surface. The groove 14 extends insubstantially the same helicoidal path as the fin l3 and results fromthe fin forming operation which will be presently described.

The tube II has walls which are thin in comparison to the tube II and issleeved within the tube l I with the outer surface in contact with theinner surface of the tube H. As shown particularly in Figure 1 of thedrawings, the outer surface of the tube "is formed with a helicallyextending rib l5 and this rib fitswithin the helical groove I to providean interlocking connection between the two tubes. As will be presentlyset forth, the rib Ii is rolled up from the inner 3 tube l2 at the sametime the material of the outer tube is extruded to form the helical finHi.

In the present instance ,the outer tube H is formed of one material andthe inner tube 12 is formed of still another material. Both materialsare preferably of non-ferrous metals, although in some cases the innertube or liner l2 may be formed of steel. In most applications the outertube H is formed of a white metal such, for example, as aluminum, andthe inner tube 12 is preferably formed of copper. These two materials,although having diflerent characteristics, are nevertheless sufiicientlysimilar to avoid appreciable relative movement by substantial changes intemperature.

The fabrication of the tube H of aluminum is advantageous not onlybecause this material has .a high thermal conducting characteristic butalso because it may be more easily worked or extruded than many othertypes of materials such as copper or steel. As a consequence, toolbreakage is reduced to a minimum and the cost of manufacture iscorrespondingly reduced. The tendency for the aluminum to corrode in thepresence of certain fluids ordinarily employed in refrigerating systems,for example, is overcome by the copper liner or inner tube l2. Also, thecopper inner tube lends itself more readily to brazing operations withsilver solder and thereby assures obtaining highly efiective jointsbetween the tubing and suitable fittings. As will be apparent from thefollowing description, the two tubes are mechanically bonded togetherduring the fin forming operation and the joint therebetween is so tightthat the loss of efiiciency in the transfer of heat is practicallynegligible. In actual practice, it has been found that the efficiencyloss through the joint between the two tubes does not exceed five percent.

Although the helical fin may be. formed on the 4 formed on the outertube H. The construction is such that the discs of the respective rollsactually track with one another and cooperate to form a continuoushelical fin on the peripheral surface of the outer tube H in a manner tobe presently described. Referring again to Figure 3 of the drawings, itwill be noted that the arbors are respectively supported on the freeends of swinging arms 2| enabling the rolls to be moved in directionstoward and away from the axis of the mandrel [8. In the presentinstance, the

. arbors 20 are driven and suitable power means (not shown) isuniversally connected to the arbors for this purpose.

The number of discs which cooperate to form each roll may varyconsiderably but for the purouter tube H by various methods anddifferent types of apparatus, nevertheless, I prefer to employ themethod and apparatus shown in Patent No. 2,508,518, dated May 23, 1950.This method and apparatus is preferred because it renders it possible todevelop a helical fin from the outer surface of the tube H withoutappreciably rubbing or bending the fin and without hardening or reducingthe ductility of the metal forming the fin.

With the above in view, reference is made more in detail to Figures 3 to6 inclusive wherein one form of apparatus is diagrammaticallyillustrated. This apparatus comprises a mandrel it having an outsidediameter approximately the same as the inside diameter of the tube l2and adapted to be sleeved into the latter. In addition, the apparatusembodies three rolls indicated generally in Figure 3 of the drawings bythe numerals ll, i8 and I9. These rolls are spaced equal distances fromeach other around the axis of the mandrel l8 and are adapted to engagethe peripheral surface of the outer tube II when the latter, togetherwith the inner tube l2, are supported on the mandrel. Each rollcomprises a series of individual circular portions or discs having hubscentrally apertured to receive an arbor 20 and having the hub portionskeyed or otherwise secured to the arbor 20. The axes of the arbors 20,or in other words the axes of the rolls, are crossed with respect to theaxis of the mandrel and are also arranged at such an angle to the axisof the mandrel as to travel a helical path about the outer tube H whichcorresponds generally to the helix angle of the fin l3 to be pose ofillustration, each roll is shown in Figure 4 of the drawings ascomprising seven discs designated by the numerals 22 to 28 inclusive.The opposite sides of the peripheral portions of the discs are relievedto provide spaces or grooves 28 between adjacent discs with convergingfin forming surfaces 30. It will further be noted from Figure 4 of thedrawings that the peripheral portions of the discs progressivelyincrease in width from the first disc 22 to the final disc 28 with theresult that the widths of the grooves 29 between the discs arecorrespondingly reduced.

Figure 8 of the drawings shows the relationship of the inner and outerplain tubes prior to subjecting the same to the fin forming operation.The outer plain tube is indicated by the numeral HA and the inner plaintube is designated by the numeral I2A. It will be noted that the outertube HA has a wall thickness exceeding twice the wall thickness of theinner tube HA, and has a length substantially less than the inner tubeHA. The purpose of such an arrangement will become more apparent as thisdescription proceeds. Briefly the outer plain tube HA elongates relativeto the inner plain tube IZA as a result of the fin rolling operation,and the wall thickness of the outer plain tube HA is sufficient to notonly permit the required degree of elongation of this tube, but to alsoprovide sufilcient material for the fins l3 while at the same timeassuring the provision of a specified wall thickness for the finishedtube II. The length differential between the plain tubes HA and HA isimportant only if it is desired to provide the tubes II and I2 withapproximately the same length after the rolling operation. In such casethe length of the inner plain tube 12A should exceed the length of theouter plain tube HA by an amount approximating the degree of elongationof the outer tube during the fin forming operation.

It is important to note that the external diameter of the inner plaintube |2A is sufiiciently less than the internal diameter of the outerplain tube HA to provide a loose fit between the plain tubes whenarranged in the telescoping relationship shown in Figure 8 of thedrawings. The

two plain tubes HA and l2A are installed upon the mandrel H5 in themanner shown in Figure 8 at the start of the forming operation. In thisconnection it is pointed out that the use of the mandrel I6 may bedispensed with providing the inner tube possesses the required rigidityto adequately support the outer tube during the fin forming operation.

After the two telescopically arranged plain tubes are properlypositioned between the fin forming rolls. the arbors 20 together withthe associated fin forming rolls are swung inwardly toward the plaintubes, and are locked in their innermost positions shown in Figure 3 ofthe drawings by any suitable means not shown herein.

discs on each roll progressively exert a reducing pressure on the outerplain tube A and contract or size the latter so that the inner surfacethereof frictionally engages the outer surface of the inner plain tubeHA. The fin forming rolls l1, l8 and iii are rotating during the aboveoperation so that rotation is imparted to both the inner and outer plaintubes.

As stated above, the discs of each fin forming roll are spaced axiallyfrom one another so that the peripheral ,portions exert acircumferential rolling pressure in a generally radially inwarddirection on axially spaced portions of the outer plain tube HA and thispressure is so determined with respect to the malleable characteristicof the outer tube A that material from the tube HA is extruded outwardlyinto the spaces between adjacent roll discs. Attention'is now called tothe fact that the axes of the rolls l1, l8 and [9 also cross the axes ofthe Plain tubes and that the discs on the several rolls operate intracking relationship. Hence the telescopically engaging tubes areadvanced by the rolls in the direction of their axes, and the peripheraledge surfaces of the roll discs apply the required circumferentialrolling pressure along ahelical path depending upon the angle at whichthe axes of the forming rolls cross the axes of the tubes. Thus thematerial progressively rolled up or extruded from the outer tube A intothe spaces between adjacent discs forms a helical fin'on the outer tubeHA.

During the rolling operation the material of the outer tube HA isextruded throughout the wall thickness of this tube so that the innersurface of the tube HA is formed with a helically extending groove l4directly opposite the root of the helical fin l3. Referring now toFigure 4 of the drawings it will be understood that as material from therelatively thick walled outer tube A is extruded outwardly into thespaces provided between adjacent roll discs, the tube i IA is reducedagainst the inner tube I2A and is elongated. Elongation of the outertube HA takes place relative to the irmer tube I2A and causes amigration of the inner surface of the outer tube along the outer surfaceof the inner tube. Since the inner surface of the outer tube HA isbrought into intimate frictional contact with the outer surface of theinner tube while this migration or elongation takes place a rubbing orscufilng action results between the outer and 0 formed of materialshaving widely varying thermal expansion characteristics. This feature isfurther assured by reason of the fact that the relatively malleableouter tube ll of the finished product not only has integral fins whichprovide a large surface area, but, in addition,-has a wall thicknessgreater than the inner tube l2. Hence even in instances where therelatively thin walled inner tube 12 is in the form of a steel tube andthe outer tube II is made of aluminum or some equivalent material, atight interlocking joint As shown in the drawings. the side surfaces 20v of the grooves 29 or spaces between the peripheral portions ofadjacent roll discs serve to guide the material extruded during therolling operation to form the fin Il and do not squeeze the materialtherebetween 'or apply appreciable axial compression to the material.Thus the material is not excessively work-hardened during the extrudingoperation and a fin ll of substantial height may be raised from theouter tube without danger of fracturing or cracking the fin. It will benoted in Figure 7 of the drawings that a slight clearance space 2| isshown at the trailing sides of the fin l2. This clearance results fromdisplacement of the material axially of the tube IIA during the finrolling operation.

inner tubes. Also at the same time the roll discs apply a radiallyinwardly directed rolling pressure on the inner tube II A along ahelical path and as a result the helically extending rib I5 is raisedfrom the outer surface of the inner tube I2A. This rib l5 projects intothe groove I4 and the migration of the grooved surface of the outer tubeHA along the ribbed outer surface of the inner tube has a peening actionon the rib which forces the same into exceptionally intimate contactwith the walls of the grooves It. In actual use this. interlockingconnection retains its tight or intimate contacting relationshipthroughout extremely wide ranges of temperature differentials and verylittle if any noticeable heat loss is apparent at the joint beween the.tubes even in cases where the tubes are During the fin rolling operationthere is a tendency for the discs to bend the fin convolutions back andforth due to interference between the sides of the discs and adjacentconvolutions of the fin l3. In order to overcome this objection whichhas a tendency to work harden the fin, the

, surfaces 30 are ground or otherwise formed to a spheroidal contour or,in other words,'are provided with a non-rectilinear curved radialcontour determined so that the peripheral portions of the discs willpass through the helicoidal path between adjacent convolutions of thefin without laterally displacing the fin or working the material to anypractical extent. There is, of course, contact between the respectiveforming discs and the material of the fin ii. In Figures 5 and 6 of thedrawings, a typical forming disc is shown in relation to the'fin and itwill be noted that the spheroidal surface 30 at one side of the disccontacts the fin at a point 32 above the axis of the tube II, and thespheroidal surface at the opposite side of the disc contacts the fin ata point 33 located below the axis of'the tube Ii. However, the curvatureof the spheroidal surfaces on opposite sides of the discs is such thatno part of either surface will either interfere with or displace theformed fin l3. Thus, regardless of the diameter of the fins beingformed, the rolls do not excessively work harden the extruded materialduring the fin forming operation, but on the other hand. merely serve toin effect guide the material as it is extruded outwardly into thegrooves 29.

Attention is further called to the fact that while the spaces or grooves29 progressively decrease in width from the first groove between theadjacent discs 22 and 23 to the last groove between the discs 21 and 22,nevertheless, no appreciable axial compression of the displaced materialtakes place because the shape of the radial surfaces 30 of the grooves29 and the depth of the latter are such that the material flowsrelatively freely into the grooves during the extruding operation. Thus,the density of the material forming the fins is very little, if any,greater than the normal density of the material prior to the fin formingoperation.

What I claim as my invention is: 1. The method 01' forming a compositetub having an integral helically extending fin projecting outwardly fromthe outer tube and having an integral helically extending rib projectingoutwardly from the inner tube into a helically extending groove formedin the inner surface of the outer tube opposite the root of the -fln,which comprises telescopically engaging inner and outer plain tubes, theinner plain tube having an external diameter sufllciently less than theinternal diameter of the outer plain tube to permit elonation of theouter tube relative to the inner tube and the outer plain tube having awall thickness substantially greater than the wall thickness of theinner plain tube, exerting a circumferential rolling pressure in aninward direction on axially spaced helically aligned portions of theouter plain tube to extrude material from the outer tube throughout itswall thickness to form an external helical fin on the outer tube and atthe same time form a helical groove in the inner surface of the outertube directly opposite the root of the helical fin, reducing the outertube against the inner tube by the circumferential rolling pressure andat the sametime elongating the outer tube relative to the inner tube tocause migration of the inner surface of the outer tube along the innertube, filling the grooves formed in the inner surface of the outer tubewith ribs rolled up from the outer surface of the inner tube bycontinued application of the rolling pressure and pressing the ribs intointimate contact with the adjacent walls of the grooves by the migrationof the outer tube along the inner tube.

2. The method defined in claim 1 in which the plain outer tube has amalleable characteristic greater than the inner tube and has an initiallength less than the length of the inner tube and in which theapplication of the circumferential rolling pressure against the outertube along a helically extending path elongates the outer tube relativeto the inner tube an amount approximating the difference in the initiallength between the two plain tubes.

WALTER P. HILL.

No references cited.

